The present invention relates to a balancing circuit for detecting unbalance between steady-state currents of semiconductor switches connected parallel to one another in an electric power converter and for balancing the steady-state currents.
FIG. 5 is a block diagram of a conventional chopper circuit including semiconductor switches, which are represented by insulated gate bipolar transistors (IGBTs), and shunt resistors for detecting the currents of the IGBTs.
Referring now to FIG. 5, a switching circuit 1.sub.1 includes an IGBT 1.sub.a1, a diode 1.sub.b1 connected in opposite parallel to the IGBT 1.sub.a1, and a gate drive circuit 1.sub.c1 connected between the gate and emitter of the IGBT 1.sub.a1 for turning on and off the IGBT 1.sub.a1. The other switching circuits are constructed in the same manner as in the switching circuit 1.sub.1. The switching circuits 1.sub.1 through 1.sub.n are connected parallel to one another to constitute a first parallel circuit. The emitters of the IGBTs 1.sub.a1 through 1.sub.an are connected collectively through respective shunt resistors 1.sub.e1 through 1.sub.en. Another parallel circuit (hereinafter referred to as a "second parallel circuit) includes switching circuits 2.sub.1 through 2.sub.n and shunt resistors 2.sub.e1 through 2.sub.en. The first and second parallel circuits thus constructed are connected in series to each other.
A DC link capacitor 5 is connected to both ends of the series circuit including the first and second parallel circuits. An inductive load 4 is connected to both ends of the second parallel circuit including the switching circuits 2.sub.1 through 2.sub.n and shunt resistors 2.sub.e1 through 2.sub.en. The terminal voltages V.sub.rs1 through V.sub.rsn of the shunt resistors 1.sub.e1 through 1.sub.en are inputted to a balancing circuit 15. The balancing circuit 15 feeds power supply voltages V.sub.cc1 through V.sub.ccn to the gate drive circuits 1.sub.c1 through 1.sub.cn of the switching circuits 1.sub.1 through 1.sub.n.
The chopper circuit of FIG. 5 regulates the electric power fed to the inductive load 4 by repeating simultaneously switching-on or switching-off of the IGBTs 1.sub.a1 through 1.sub.an by means of a pulse distributor circuit 3. FIG. 8 is a set of operational wave forms of the chopper circuit of FIG. 5. Referring now to FIG. 8, V.sub.CE indicates the voltage between the collector and emitter of the IGBT, I.sub.con indicates the turn-on collector current of the IGBT, I.sub.c1 through I.sub.cn indicate the collector currents of the respective IGBTs 1.sub.a1 through 1.sub.an, and I.sub.o indicates a load current.
A plurality of switching circuits (IGBTs) are connected parallel to obtain a current capability more than the rated current of one single switching circuit.
The chopper circuit of FIG. 5 operates as follows. The load current I.sub.o flows through the inductive load 4 when the IGBTs 1.sub.a1 through 1.sub.an are turned on. When the IGBTs 1.sub.a1 through 1.sub.an are simultaneously turned off from the above described state, the current I.sub.o that has been flowing through the inductive load 4 circulates through the diodes 2.sub.b1 through 2.sub.bn. Thus, by repeating the turning-on and turning-off of the main switches, i.e. IGBTs 1.sub.a1 through 1.sub.an, the electric power to be fed to the load is regulated. The second parallel circuit including the switching circuits 2.sub.1 through 2.sub.n and shunt resistors 2.sub.e1 through 2.sub.en is provided to circulate energy stored in the inductive load 4 while the IGBTs 1.sub.a1 through 1.sub.an are in their on-state.
The conventional chopper circuit including a plurality of semiconductor switches connected parallel to one another causes the following problems. The currents flowing through the IGBTs 1.sub.a1 through 1.sub.an are expressed by the following formula (1) when all the currents are balancing to one another. ##EQU1##
FIG. 7 is a pair of curves relating to the collector current and the saturation voltage between the collector and emitter of the IGBT. Unbalance is caused between the currents 1.sub.c1 through 1.sub.cn flowing through the IGBTs 1.sub.a1 through 1.sub.an as described in FIG. 8 due to the distribution or unbalance of each saturation voltage between the collector and emitter of each IGBT shown in FIG. 7 and the distribution or unbalance of the circuit constants caused by wiring. The unbalance between the currents of the IGBTs causes unbalance between the steady-state losses and, in the worst case, it causes breakdown of the switching devices.
To obviate the above described problems, the current flowing through each switching device is detected, and the voltage between the gate and emitter of each switching device is boosted or lowered depending on the values of the detected current, so that the characteristics between V.sub.CE and I.sub.c may be identical to one another.
FIG. 7 shows an example in which the voltage between the gate and emitter of the IGBT 1.sub.a2 is changed from V.sub.GE1 to V.sub.GE2 so that the characteristics of the IGBT 1.sub.a2 may be identical to those of the IGBT 1.sub.a1.
In FIG. 5, the shunt resistors 1.sub.e1 through 1.sub.en are disposed to detect the currents 1.sub.c1 through 1.sub.cn flowing through the IGBTs 1.sub.a1 through 1.sub.an. The current flowing through each switching device is detected from the following formula (2), which relates to the voltage across the switching device and the current flowing through each switching device. In the formula (2), R represents the resistance in the shunt resistors 1.sub.e1 through 1.sub.en. EQU V.sub.rsi =R.times.I.sub.ci (i=1, 2 . . . n) (2)
However, as the rated value of the switching device increases, the shunt resistor with a larger power capacity should be employed. The shunt resistor with a larger electric capacity causes larger size of the power converter.
FIG. 6 is a block diagram illustrating another conventional technique for detecting the currents flowing through the switching devices. Referring now to FIG. 6, a sensing resistor 1.sub.e1 is connected between a sensing terminal and an emitter terminal of an IGBT 1.sub.a1 to detect the terminal voltage V.sub.s. In FIG. 6, 1.sub.1 ' and 2.sub.1 ' designate switching circuits.
The circuit of FIG. 6 detects the currents flowing through the switching devices as follows. When a current I.sub.c1 flows in association with the turning-on of the IGBT 1.sub.a1, a current I.sub.s (hereinafter referred to as a "sensing current") as high as one several ten thousandth of the current I.sub.c1 flows. As the sensing current I.sub.s flows, a sensing voltage V.sub.s is generated across the sensing resistor 1.sub.e1. The sensing voltage V.sub.s increases with an increase of the collector current I.sub.c1. The sensing voltage V.sub.s, the sensing current I.sub.s and the resistance R.sub.s of the sensing resistor 1.sub.e1 are related by the following formula (3). EQU V.sub.s =R.sub.s .times.I.sub.s ( 3)
In the conventional technique described in FIG. 6, the sensing current I.sub.s flowing through the sensing resistor 1.sub.e1 is from several to several tens mA. Therefore, the sensing technique that employs a sensing resistor facilitates to minimize the power converter more than the sensing technique that employs a shunt resistor.
However, the sensing voltage V.sub.s across the sensing resistor causes a large distribution or unbalancing as compared with the sensing voltage obtained by the shunt resistance. Therefore, in the power converter like the chopper circuit of FIG. 5 which includes a plurality of switching circuits connected parallel to one another, sufficiently precise balance control is not attained in balancing the currents by detecting the currents flowing through the respective switching devices with the sensing resistors.
In view of the foregoing, it is an object of the invention to provide a balancing circuit for balancing the steady-state currents of the switching devices in an electric power converter, wherein the size of the power converter are minimized, and the steady-state currents are precisely balanced.